Examensarbeiten in der Abteilung Spektroskopie
Efficient emitters for THz radiation
Generating electromagnetic radiation at THz frequencies has always been more challenging than in other frequency bands. Our goal is to improve the performance of current THz emitters and to design, fabricate and test new photoconductive THz emitters. Smallest features of these emitter device structures are µm and sub-µm. For device fabrication we use electron beam and photo lithography techniques. Device characterization makes use of ultrafast spectroscopic techniques with femtosecond lasers. Students interested in conducting research / pursuing a master thesis will participate in the following topics:
• Optimizing material properties for THz device applications. Typical materials for THz emitters include semiconductors and nonlinear optical crystals: GaAs, InGaAs, Ge, Si, ZnTe, GaP, etc.
• Improving antenna structures and overall device layout for more efficient THz emission.
Contact: Dr. Stephan Winnerl
Graphene related materials for optoelectronics
Monolayer graphene and bilayer graphene are semimetals with zero band gap. In bilayer graphene, however, a small band gap can be opened by applying an electric field perpendicular to the layer. Graphene of high structural quality features very high carrier mobilities, in particular, when the material is charge neutral.
For experiments with ultrashort laser pulses, well characterized graphene monolayer and bilayer samples with different doping levels are needed. The main tools for characterization are Raman spectroscopy and Fourier-transform infrared spectroscopy.
Graphene offers various mechanisms that can be exploited for photodetection. Particularly strong photocurrents, exceeding one electron per photon, have been demonstrated for asymmetrically illuminated free-standing graphene flakes in magnetic fields. This is a hint for carrier multiplication at the edges of the sample. We intend to perform experiments on large-area samples of high quality that will enable to characterize the detection process not only in the visible spectral range but also in the mid-infrared and terahertz range.
Contact: Dr. Stephan Winnerl
Growth and properties of III-V semiconductor nanowires
III-V compound semiconductors are a class of materials with outstanding electronic and optoelectronic properties (high electron mobility, wide range of direct energy band gaps, etc.). High-speed transistors, efficient light emitting diodes, and solid-state lasers are only some of the devices widely used in our everyday lives, e.g. in smart phones, blu-ray technology, wireless local-area networks, LED displays, etc. In the era of nanotechnology, we focus our research on new physical phenomena that occur in III-V nanostructures. Such phenomena may enhance the performance of the aforementioned devices or open ways to novel device architectures. Students interested in conducting research / pursuing a master thesis in our laboratory would be involved with one or more of the following topics:
• growth of III-As (arsenide) nanowires and thin films by molecular beam epitaxy
• investigation of their structural, electrical, and optical properties using x-ray diffraction, Hall-effect, photoluminescence, etc.
• investigation of the growth mechanisms using in-situ reflectometry
Contact: Dr. Emmanouil Dimakis